1. Field of the Invention
The present invention relates to an improved process for preparing phenols.
2. Description of the Related Art
The process of acid-catalyzed cleavage of cumene hydroperoxide into phenol and acetone has been of particular industrial importance for a long time. In the preparation of phenol from cumene by the Hock process, cumene is oxidized to cumene hydroperoxide (CHP) in a first reaction step, known as oxidation, and the CHP is subsequently concentrated to from 65 to 90% by weight in a vacuum distillation, known as concentration. In a second reaction step, known as cleavage, the CHP is cleaved into phenol and acetone by action of an acid, usually sulfuric acid. In this step, the dimethyl phenyl carbinol (DMPC) formed in the oxidation is partly cleaved in an equilibrium reaction into α-methylstyrene (AMS) and water, while a further part of the DMPC reacts with CHP to form dicumyl peroxide (DCP); the rest remains in the cleavage product. After neutralization of the cleavage product, this product mixture is usually worked up by distillation.
In the cleavage, part of the AMS or of the DMPC forms high boilers (dimers, cumylphenols, bisphenols) which are discharged as residue in the distillation. The AMS still present after the neutralization, is hydrogenated to cumene in the distillation and is returned to the oxidation. DMPC which is not reacted in the cleavage ends up as high boiler in the residue; part of it reacts further in the hot phenol columns to form AMS from which high-boiling secondary components are once again formed. DCP is stable at customary cleavage temperatures (50-70° C.). It decomposes thermally in the hot phenol columns forming, in our experience, o-cresol, at least in part. On the other hand, in the presence of acid, DCP can be cleaved into phenol, acetone and AMS at temperatures above 80° C. It is therefore obvious for the remaining DMPC and the DCP formed in the cleavage to be reacted completely immediately after the cleavage by means of a targeted increase in the temperature in the presence of the acid used as catalyst in the cleavage. In this way, DMPC is largely converted into AMS and DCP is converted virtually completely into phenol, acetone and likewise AMS.
Such a thermal after-treatment of the cleavage product has already been described in U.S. Pat. No. 2,757,209, where temperatures above 100° C., specifically from 110 to 120° C., were employed. The objective of this thermal after-treatment was the complete dehydration of DMPC to AMS. On the other hand, U.S. Pat. No. 4,358,618 describes a thermal after-treatment which has the aim of converting all of the DCP formed in the cleavage into phenol, acetone and AMS; in that patent, temperatures of 120 and 150° C. are employed. U.S. Pat. No. 5,254,751 describes a thermal after-treatment which has the same objective as that in U.S. Pat. No. 4,358,618 and uses temperatures of from 80 to 110° C. Finally, in DE 197 55 026 A1, the after-treatment is carried out in a temperature range above 150° C. In all these processes known from the prior art, the thermally treated product is subsequently cooled to (customarily) 40° C. by means of a cooler, then neutralized and, after separating off a salt-containing aqueous phase, worked up by distillation.
A disadvantage of the above-described processes is that hydroxyacetone and other carbonyl compounds such as acetaldehyde, propionic aldehyde and phenyl propanal are formed as by-products and these, firstly, make the work-up of the reaction product difficult and, secondly, hydroxyacetone in particular reacts with phenol in specific phenol purification processes to form high boilers, thus leading to undesirable losses of phenol. It would therefore be desirable to reduce the content of hydroxyacetone and other impurities in the cleavage product.
U.S. Pat. No. 6,066,767 describes a process for removing hydroxyacetone and other carbonyl compounds from the product of the cleavage of cumene hydroperoxide. For this purpose, the reaction product of the cumene hydroperoxide cleavage is extracted with an aqueous salt solution in a temperature range of 15-80° C. to remove hydroxyacetone, inter alia. The loaded extractant is subsequently treated with a base in a separate reactor to convert hydroxyacetone into condensation products. The extractant which has been treated in this way is returned to the extraction stage where the condensation products go into the organic phase and are then separated off in the work-up of the phenol- and acetone-containing organic phase. The examples show that, despite the very complicated apparatus employed for purification by extraction and subsequent reaction of the extracted hydroxyacetone, the organic product phase which is passed to further work-up for the isolation of phenol still contains 500-800 ppm of hydroxyacetone.
Furthermore, it has been discovered that the sodium hydroxide still present in the extractant will react with the phenol present in the organic phase of the cleavage product when the extractant is returned to the extraction stage. The basic strength of sodium phenolate formed thereby is too low to achieve a reasonable reaction rate for the conversion of hydroxyacetone in the aqueous phase after extraction. Therefore it is mandatory in the process disclosed in U.S. Pat. No. 6,066,767 to add fresh sodium hydroxide to the stage where the hydroxyacetone is converted to high-boilers. Thus an additional disadvantage of the process disclosed in U.S. Pat. No. 6,066,767 is, that fresh starting material like sodium hydroxide is consumed and wasted to a considerable extent contributing to undesired high operating costs for the prior art process.
In Vasileva, I. I. et al., 2000 Neftepererab. Neftekhim., Moscow, Russ. Fed., 12:34-38 a process of extraction and conversion of hydroxyacetone from the product of cumene hydroperoxide cleavage similar to U.S. Pat. No. 6,066,767 is disclosed. The only disclosure additional to the teaching of U.S. Pat. No. 6,066,767 is that air can be introduced into the reactor where the aqueous extractant phase comprising hydroxyacetone is treated with added sodium hydroxide in order to increase the rate of conversion of hydroxyacetone. Therefore the process taught by Vasileva et. al. exhibits the same disadvantages as the process known from U.S. Pat. No. 6,066,767.
From US 2002-0183563 a process for preparing phenols, in which the pH of the reaction product from the acid-catalyzed cleavage of alkylaryl hydroperoxides is set to a value of at least 8 by addition of an basic aqueous solution at a temperature of at least 100° C. prior to the work-up of the product is known. Thereby an emulsion of the basic aqueous solution in the organic phase of the cleavage product is formed with the result that a high volume flow of a two-phase inhomogeneous mixture like in the process of U.S. Pat. No. 6,066,767 has to be processed. Furthermore, it has been discovered that under the reaction conditions described in US 2002-0183563 condensation products with acetone are formed resulting in losses of valuable products.
U.S. Pat. No. 4,283,568 describes a process for the recovery of phenol from a reaction mixture resulting from the acid cleavage of cumene hydroperoxide. Neutralization of the acidic reaction mixture is affected with an aqueous solution of sodium phenolate that is obtained a recycle stream in subsequent work-up stages of the process. The amount of added aqueous phenolate solution is such that a heterogeneous two-phase mixture is formed and the aqueous phase has to be separated from the organic phase in a settler.
In the process described in U.S. Pat. No. 3,692,845 non-aqueous component comprising a polyamine compound is added to the phenol obtained by the acid cleavage of cumene hydroperoxide in order to remove carbonyl bearing impurities.